Notify event base if there are no more events, so it can exit without delay

Fixes: #623
(cherry picked from commit 23c2914f6b430f2c2d74c267c13ffab3dda1b325)

event.c

@@ -2847,6 +2847,10 @@ event_del_nolock_(struct event *ev, int blocking)
 			notify = 1;
 			res = 0;
 		}
+		/* If we do not have events, let's notify event base so it can
+		 * exit without waiting */
+		if (!event_haveevents(base) && !N_ACTIVE_CALLBACKS(base))
+			notify = 1;
 	}
 
 	/* if we are not in the right thread, we need to wake up the loop */

test/regress.c

@@ -1039,6 +1039,34 @@ test_del_wait(void)
 	end:
 	;
 }
+
+static void null_cb(evutil_socket_t fd, short what, void *arg) {}
+static void* test_del_notify_thread(void *arg)
+{
+	event_dispatch();
+	return NULL;
+}
+static void
+test_del_notify(void)
+{
+	struct event ev;
+	pthread_t thread;
+
+	test_ok = 1;
+
+	event_set(&ev, -1, EV_READ, null_cb, &ev);
+	event_add(&ev, NULL);
+
+	pthread_create(&thread, NULL, test_del_notify_thread, NULL);
+
+	{
+		struct timeval delay = { 0, 1000 };
+		evutil_usleep_(&delay);
+	}
+
+	event_del(&ev);
+	pthread_join(thread, NULL);
+}
 #endif
 
 static void
@@ -3452,6 +3480,7 @@ struct testcase_t main_testcases[] = {
 #ifdef EVENT__HAVE_PTHREADS
 	/** TODO: support win32 */
 	LEGACY(del_wait, TT_ISOLATED|TT_NEED_THREADS),
+	LEGACY(del_notify, TT_ISOLATED|TT_NEED_THREADS),
 #endif
 
 	END_OF_TESTCASES